1. Field of the Invention
The present invention relates to receiving circuits, more particularly, to a receiving circuit used in a wireless communication apparatus.
2. Description of the Background Art
With reference to the drawing, a receiving circuit used in a conventional wireless communication apparatus will be described below. The conventional wireless communication apparatus includes a mobile phone and a PHS, for example. Here, FIG. 8 is a block diagram showing a structure of the receiving circuit of the conventional wireless communication apparatus.
The receiving circuit of the wireless communication apparatus as shown in FIG. 8 includes an antenna 111, an amplifier 113, a bandlimiting filter 114, a frequency conversion circuit 115, a local oscillator 116, a bandlimiting filter 117, a frequency conversion circuit 118, and a local oscillator 119. Hereinafter, an operation of the above-described receiving circuit of the wireless communication apparatus will be briefly described.
First, a high-frequency signal is received by the antenna 111. After the received high-frequency signal is amplified by the amplifier 113, the amplified high-frequency signal passes through the bandlimiting filter 114, which is designed to pass only a required signal band, and is inputted to the frequency conversion circuit 115. Next, the high-frequency signal is mixed with a first local oscillating signal, which is outputted from the local oscillator 116, by the frequency conversion circuit 115. As a result, the high-frequency signal is converted to a first intermediate frequency signal. Then, the first intermediate frequency signal is inputted to the frequency conversion circuit 118 after passing through the bandlimiting filter 117. The first intermediate frequency signal is mixed with a second local oscillating signal, which is outputted from the local oscillator 119, by the frequency conversion circuit 118. As a result, the first intermediate frequency signal is converted to a second intermediate frequency signal. Then, various processes are performed for the second intermediate frequency signal by a circuit connected to a subsequent stage. As a result of the above-described processes, the high-frequency signal is converted to the second intermediate frequency signal.
When a user travels with the above wireless communication apparatus and approaches a base station, the wireless communication apparatus receives a high-frequency signal with a high electric field strength. In this case, a signal level of the received high-frequency signal substantially exceeds an output dynamic range of the frequency conversion circuit 115. As a result, the frequency conversion circuit 115 operates in the saturation region, thereby deteriorating reception characteristics of the receiving circuit of the wireless communication apparatus.
In order to solve the above-described problem, there exists a receiving circuit of a wireless communication apparatus as shown in FIG. 9. The receiving circuit of the wireless communication apparatus additionally includes a variable attenuator 112 between the antenna 111 and the amplifier 113. The feedback control is performed to control the amount of attenuation in the variable attenuator 112 in accordance with a signal level of a reception signal. As a result, it is possible to prevent a signal having a signal level substantially exceeding a dynamic range of the frequency conversion circuit 115 from being inputted to the frequency conversion circuit 115. Hereinafter, with reference to FIG. 9, the above receiving circuit of the wireless communication apparatus will be described.
The receiving circuit of the wireless communication apparatus includes the antenna 111, the variable attenuator 112, the amplifier 113, the bandlimiting filter 114, the frequency conversion circuit 115, the local oscillator 116, the bandlimiting filter 117, the frequency conversion circuit 118, the local oscillator 119, and a gain control section 121. Hereinafter, an operation of the receiving circuit of the wireless communication apparatus will be briefly described.
First, the operations performed by the antenna 111, the amplifier 113, the bandlimiting filter 114, the frequency conversion circuit 115, the local oscillator 116, the bandlimiting filter 117, the frequency conversion circuit 118, and the local oscillator 119 are identical to the operations performed by their counterparts in the receiving circuit as shown in FIG. 8, with the descriptions thereof omitted.
The second intermediate frequency signal outputted from the frequency conversion circuit 118 is inputted to the gain control section 121. The gain control section 121 rectifies the second intermediate frequency signal to obtain a direct current signal. Here, the variable attenuator 112 controls the amount of attenuation by using the obtained direct current signal as a gain control signal. Specifically, the variable attenuator 112 increases or decreases the amount of attenuation in accordance with a level of the gain control signal. Thus, the receiving circuit is able to vary a level of a reception signal.
As such, in the case where a reception signal with high electric field strength is inputted or a disturbing signal within a band of the bandlimiting filter 117 whose band is narrower than that of the bandlimiting filter 114 is inputted, a signal level of the second intermediate frequency signal is increased due to control of the variable attenuator 112, and a direct voltage (a gain control signal) is increased accordingly. As a result, the amount of attenuation in the variable attenuator 112 is increased, and a dynamic range of the frequency conversion circuit 115 is ensured, thereby preventing the frequency conversion circuit 115 from operating in the saturation region (for example, see Japanese Laid-Open Patent Publication No. H10-126301).
Note that, other than the above-described invention, there exists a receiving circuit of a wireless communication apparatus as disclosed in Japanese Laid-Open Patent Publication No. H10-93367 or Japanese Laid-Open Patent Publication No. H5-335857.
However, the receiving circuit as shown in FIG. 9 has the following problem. The gain control section 121 detects a level of an output signal outputted from the frequency conversion circuit 118, and performs AGC (Automatic Gain Control). Thus, in the case where a disturbing signal having a high signal level lying outside the band of the band limiting filter 117 is received, the gain control section 121 does not perform an AGC operation, which will be described below in detail with reference to the drawing. FIG. 10 is an illustration showing a high-frequency signal including a disturbing signal. Specifically, a horizontal axis represents a frequency, and a vertical axis represents a signal level.
In general, the amplifier 113 and the frequency conversion circuit 115 are required to deal with a plurality of signals having respective frequency bands. Specifically, the bandlimiting filter 114 allows signals having respective frequency bands f1 to f3 as shown in FIG. 10 to pass therethrough.
On the other hand, the bandlimiting filter 117 extracts only a required reception band. Specifically, in the case where signals as shown in FIG. 10 are inputted, the bandlimiting filter 117 passes only a required signal (frequency f2). Thus, in the case where a high-frequency signal including a disturbing signal (frequency f3), whose signal level is higher than the required signal as shown in FIG. 10, is received by the receiving circuit, only the required signal (frequency f2) is outputted to the gain control section 121. In this case, gain control should be performed based on the signal level of the disturbing signal (frequency f3) having the highest signal level. However, gain control is performed based on the intensity of the required signal (frequency f2) whose signal level is lower than the disturbing signal (frequency f3). As a result, a high-frequency signal including a disturbing signal (frequency f3) which is not adequately attenuated is inputted to the frequency conversion circuit 115, whereby a reception performance of a receiver is deteriorated since the frequency conversion circuit 115 operates in the saturation region.
In addition to the above-described problem, the receiving circuit as shown in FIG. 9 makes it difficult to reduce power consumption, which will be described in detail below.
For example, in the case where the wireless communication apparatus lies near the base station, an electric field strength of the required signal becomes relatively high. In this case, a level of the reception signal which has passed through the attenuator 112, the amplifier 113, and the frequency conversion circuit 115 does not change.
However, in the receiving circuit as shown in FIG. 9, even if an electric field strength of the required signal is substantially high, the amplifier 113 amplifies the reception signal with a predetermined constant gain and current value. As a result, in the receiving circuit as shown in FIG. 9, even if there is no need to perform an amplification process due to a sufficiently high electric field strength, the amplifier 113 performs amplification using a constant gain and current, which results in unnecessary power consumption in the receiving circuit.
Also, in addition to the above-described two problems, in the receiving circuit as shown in FIG. 9, the reception sensitivity of the receiving circuit is lowered due to the variable attenuator 112 inserted between the antenna 111 and the amplifier 113.
As shown in FIG. 9, in the conventional receiving circuit, the variable attenuator 112 is positioned between the antenna 111 and the amplifier 113. In this case, a signal loss of up to about 0.5 dB occurs in the variable attenuator 112 due to an insertion loss of a part itself and a loss resulting from the insertion of a part in a signal transmission line. Such a signal loss will result in a low SNR (Signal Noise Ratio). Especially, it is rather difficult to compensate for low SNR caused in a circuit preceding the amplifier 113 by using a circuit subsequent to the amplifier 113. Such a problem will be described below in detail. In the following descriptions, the total NF (Noise Figure) (hereinafter, referred to as NFtotal) of a plurality of circuits in a cascade arrangement is used as one example. NF represents the ratio of SNR of an input signal to SNR of an output signal. Specifically, NF is represented as the following equation:NF=(Sin/Nin)/(Sout/Nout).
In the above-described case, NFtotal is given by the following equation: NFtotal=NF1+(NF2−1)/G1+(NF3−1)/(G1*G2)+ . . . . Note that NF1 represents an NF of a first stage circuit, and G1 represents a gain of the first stage circuit. Also, NF2 represents an NF of a second stage circuit, and G2 represents a gain of the second stage circuit. Still further, NF3 represents an NF of a third stage circuit, and G2 represents a gain of the third stage circuit.
Here, assuming that the total NF of a circuit in which no loss occurs in a previous stage of the amplifier is NFtotal1, the following equation is established.NFtotal1=NF1+(NF2−1)/G1+(NF3−1)/(G1*G2)+
On the other hand, assuming that the total NF of a circuit in which a loss occurs in a previous stage of the amplifier is NFtotal2, the following equation is established. Note that NF0 represents an NF of the previous stage circuit of the amplifier, and G0 represents a gain of the previous stage circuit of the amplifier.NFtotal2=NF0+(NF1−1)/G0+(NF2−1)/(G0*G1)+(NF3−1)/(G0*G1*G2)+
Here, assume that a loss in the previous stage of the amplifier is 0.5 dB. In this case, NF0=0.5 dB, and G0=−0.5 dB. Thus, in the circuit in which a loss occurs in the previous circuit of the amplifier, not only NF0 is added but also a term NF1 is incremented since G0 is equal to or smaller than 1, which results in low NFtotal2. Even if an NF in a subsequent circuit is improved in order to prevent NFtotal2 from being lowered, the improvement effect of NFtotal2 of the entire circuit substantially reduces since the improved NF is multiplied by 1/(G0*G1).
As such, a loss in a previous stage of the amplifier 113 results in a low SNR, and it is difficult to compensate for such a low SNR in a subsequent stage of the amplifier 113. As a result, the reception sensitivity of the wireless communication apparatus is substantially lowered.